Hi,
I'm not sure to clearly visualize what you want to achieve, as the main benefit for MLAG is to have dual-attached L2 devices. And across a WAN... not sure to understand.
Anyway, I've asked around and if this is a direct link between the MLAG peers, distance shouldn't matter. I'm assuming failure of that WAN link would be just a regular link failure and detectable by the MLAG peers. I don't expect QA to have done long distance testing, but in theory, not an issue.